High specific capacitance and high energy density supercapacitor electrodes enabled by porous carbon with multilevel pores and self-doped heteroatoms derived from Chinese date

Diamond & Related Materials 97 (2019) 107455

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High specific capacitance and high energy density supercapacitor electrodes enabled by porous carbon with multilevel pores and self-doped heteroatoms derived from Chinese date

T

YanLei Zhang , Chen Sun, ZhiShu Tang ⁎

⁎

Shaanxi University of Chinese Medicine, Shaanxi Collaborative Innovation Center of Chinese Medicine Resources Industrialization, State Key Laboratory of Research & Development of Characteristic Qin Medicine Resources (Cultivation), Shaanxi Innovative Drug Research Center, Xianyang 712083, PR China

ARTICLE INFO

ABSTRACT

Keywords: Chinese date Porous carbon High energy density Supercapacitors

Supercapacitors are attracting us for having advantages of fast charging and discharging, fine power density and great stability, however they are suffered from lower energy density, and high capacitance materials are the main point for fabrication of high energy density electrode materials. We present here a novel porous carbon derived from a high yield biomass Chinese date using a simple pre-carbonization process combined with KOH activation. The porous carbon prepared under the optimized conditions mainly composed of micropores and a certain number of mesopores, and its surface is covered with irregular pits, together with 18.13 at.% and 0.57 at. % self-doped O and N. The mesopores are conducive to the rapid transfer and diffusion of electrolyte ions; while the existence of micropores and irregular pits contribute to the large specific surface areas for ions to accommodate, and the participation of heteroatoms especially O 1 s, promote the wettability of electrolyte to electrode materials. It is just because of a combination of these advantage factors together, thus the electrodes based on this material generated an ultrahigh specific capacitance of 518 F/g at 0.5 A/g, with excellent rate performance of 40 F/g at 70 A/g. Notably, symmetric supercapacitors based on this material exhibited energy density of 18.5 Wh kg−1 at a power density of 373.8 W kg−1 in the aqueous electrolyte Na2SO4, and a high specific energy density of 51.3 Wh kg−1 was achieved at a high power density of 767.8 W kg−1 in the organic electrolyte Et4NBF4. These excellent results verified the potential of Chinese date derived porous carbon for high performance supercapacitors, and due to the advantages of abundant, renewable and inexpensive raw material, and eco-friendliness simplicity preparation process, it is practical to industrialization this material in the near future for scale-up production of high energy supercapacitors.

1. Introduction

density supercapacitors. For the specific capacitance, however, the factors mainly come from the electrode materials, an ideal electrode material should not only have a high specific capacitance but also be able to withstand a high voltage, only that will generate a high energy density. Take all of these together, the key to develop high energy density supercapacitors is to develop suitable electrode materials. Supercapacitors are divided into two categories of pseudocapacitors and electrical double-layer capacitors (also known as EDLCs.) depending on their energy storage mechanisms [5]. Charge-storage of pseudocapacitors is through reduction-oxidation reactions and intercalation processes, therefore, the presence of metals or heteroatoms is generally required, usually generate ultrahigh specific capacitances, [6] however with poor electrical conductivity thus resulted in bad durable stability, as a result the practical application of this kind of supercapacitor is also limited. [7] The charging process of EDLCs is more

Supercapacitors are fascinating us for having the advantages of rapid power transmission, fast charging and discharging capability, higher power density, and good cycle stability, thus they are expected to replace batteries in the future [1]. However, the relatively low energy density greatly hindered there practical utilization in compact and portable electric devices [2]. According to the equation of E= 1 CV2, the 2 energy density is proportional to the specific capacitance and the square of the voltage range [3]. Therefore, to improve the energy density, the specific capacitance and voltage range should be improved. Of which, the voltage range is mainly depended on electrolytes, aqueous electrolytes as Na2SO4 can bear maximum higher voltage of 2 V, while organic electrolytes as Et4NBF4 can afford voltage of 3 V or higher [4]. Therefore using these electrolytes is a good choice for high energy ⁎

Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Tang).

https://doi.org/10.1016/j.diamond.2019.107455 Received 25 March 2019; Received in revised form 16 June 2019; Accepted 17 June 2019 Available online 19 June 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.

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physically, during the charging process, the electrolyte ions are adsorbed on the surface of the electrode materials [8]. Thus for high performance EDLCs, the electrode materials should have large surface areas for electrolytes ions to stay, also should have proper size of pores for them to passage quickly to their destinations [9]. And need to say that existence of heteroatoms such as N, O, S and B could help the wettability of electrolyte to electrodes surfaces to reduce resistance and also could generate pseudocapacitances, therefore will greatly benefit the specific capacitance [10,11]. Various kinds of carbon based materials have been developed to fulfil the requirements of high performance supercapacitors [12–20]. Among them, in recent years, porous carbon derived from biomass and biomass waste has attracted a lot of attention [21]. As raw materials are cheap and easy to obtain, the preparation process is simple, and the performance is excellent, so it has been widely studied and applied in supercapacitors [22–26]. The raw materials are not difficult to find, and the preparation processes are similar to each other. Normally by first carbonized under hydrothermal or calcination to form biochars, then is activated with or without activators under higher temperatures [27–35]. There are also some novel biomass selected as precursors to prepare supercapacitor materials. Recently, albizia flowers were used as precursor to prepare porous carbon by pyrolysis combined with KOH activation, the porous electrode material with a high specific surface areas of 2757.63 m2 g−1 and high self-doping N contents, exhibited a high energy density of 26.3 W h kg−1 at a power density of 429 W kg−1 in Na2SO4 electrolyte [36]. Another example is Cicada slough, which is the skin and shell of cicada larvae and is also a traditional Chinese medicine, by carbonized at 600 °C together with KOH activation at 650 °C, the resultant activated carbon has a high content of 12.06 at.% of N, O, S, and P co-doped, exhibited a energy density of about 9 Wh kg−1 in aqueous electrolyte [37]. These are just a few of them. Other biomass such as elm samara [38] and garlic skin [39] are also have been used to prepare high performance material for supercapacitors. The reasons for these biomass derived porous carbons to show excellent performances is that the biomass themselves may contain some unique hierarchical structures as maize derived popcorn, which is maintained during the process of carbonization and activation; In addition, some biomass as cashmere and Cicada slough themselves contain a large number of active compounds composed of heteroatoms, which formed self-doped porous carbon materials after carbonization and activation, and these are just the factors necessary to generate high performance for supercapacitors. In addition biomass and biomass waste are also ideal materials for preparation of aerogels or complexes with metals, which after further processes then can be used as electrodes for supercapacitors [40–43] with high performance. Compared with those non-biomass derive porous carbons requires templates and other complex preparation processes [44–47], these biomass derived materials with advantageous of high performance and convenient preparation processes. These all put together proves the efficient of biomass in preparing porous carbons for high performance supercapacitors. Chinese date (CD) is one of the most famous Chinese medicines and is also a delicious fruit. It is mainly composed of polysaccharide, and also rich in protein, fat, sugar, 4 carotene, vitamins B, vitamin C, vitamin P, calcium, phosphorus, iron and cyclic adenosine phosphate and other nutrients, and has been proved to have a variety of beneficial effects on the human body. This plant is also highly adaptable to the environment, resistant to cold and drought, and has the function of preventing wind and fixing sand. Therefore, it is widely distributed in the northern part of China, both wild and cultivated, especially in the western arid regions of Shaanxi, Shanxi, Hebei and Xinjiang. The quality of Chinese date produced in Xinjiang is the best, which greatly impacts the market of Chinese date produced in other regions. In addition, the production of Chinese date is very high, but the market

demand is limited, so every year will encounter a bumper harvest but an unmarketable situation. In northern of Shaanxi, a large number of dates are abandoned every year because the market profit does not cover the cost of harvesting. So the mountains and fields are littered with abandoned dates, some used as animal feed, but much of it is left to rot as waste. This is a waste of resources, and because of the high sugar content, during the decay process will produce a bad smell, resulting in environmental pollution either. Thus it is urgent for us to develop other utilization strategies to deal with this situation. The Chinese date produced in northern of Shaanxi is high yield and cheap to get and is mainly composed of polysaccharides and some other active ingredients, so it has very high carbon content, as well as a large amount of O and other elements. Therefore Chinese date is expected to be an ideal raw material for preparation of porous carbon for high performance supercapacitors. In this study, for the first time we found that by calcination combined with KOH activation, Chinese date can be turned to porous carbon as ideal electrode materials for high performance supercapacitors. Through the regulation of activation temperatures and the amount of activator, the porous carbon DC600-4 derived from Chinese date under the optimized conditions with a high specific surface areas of 1940.7 m2 g−1, and a unique structure composed of micropores and a certain amount of mesopores, together with 18.13 at. % and 0.57 at.% self-doped O and N. These advantages made the electrodes based on this material generate an ultrahigh specific capacitance of 518 F/g at 0.5 A/g, with excellent rate performance of 40 F/g at 70 A/g. Notably, symmetric supercapacitors based on this material exhibited high energy density of 18.5 Wh kg−1 at a power density of 373.8 W kg−1 in the aqueous electrolyte Na2SO4, and a remarkable high specific energy density of 51.3 Wh kg−1was achieved at a power density of 767.8 W kg−1 in the organic electrolyte Et4NBF4. Thus the Chinese date derived porous carbon holds the great potential as electrodes for high capacitance and high energy density supercapacitors. 2. Experimental section 2.1. Materials The mature Chinese dates were collected from Yan'an, Shaanxi Province in October 2018. After cleaning, they were dried at 80 °C for use. Potassium hydroxide and hydrochloric acid were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The chemicals are in analytic grade and used without further purification. Ultra-pure water is used throughout the whole process. 2.2. Material preparation The cleaned and dried Chinese date was calcined at 700 °C under N2 for 2 h to obtain pre‑carbonized product DBC, which was then fully ground and mixed with KOH at different proportions of 1:3, 1:4, and 1:5 (DBC to KOH) in an agate mortar. The mixtures were then translated to quartz boats and loaded in a tube furnace, heated at a rape rate of 5 °C/min to 500 °C, 600 °C, and 700 °C under N2 (200 mL/min) respectively, and maintained at the final temperatures for 2 h. After cooling naturally to room temperature, the mixtures were washed with hot hydrochloric acid solution and ultra-pure water thoroughly to remove the impurities and dried at 80 °C overnight, the final products were marked as DCt-p, in which t and p represent the activation temperature, and the ratio of KOH to DBC, respectively. 2.3. Materials characterization The microstructures and morphology of the samples were characterized by scanning electron microscopy (SEM, Hitachi SU-70), high resolution transmission electron microscopy (HRTEM, JEOL 2100F), X-

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ray diffraction (XRD, Philips X using Cu kα Radiation), Raman spectra were performed with a LabRAM HR800 from JY Horiba, and pore structure measurement was processed using nitrogen isothermal adsorption method (ASAP 2020), Brunauer-Emmett-Teller (BET) method was used to calculate the surface area, and the pore size distributions plots were obtained from the desorption of the isotherms using the tplot model. The surface elements analysis of the carbon samples were processed by an X-ray photo electron spectrometer (XPS) method. 2.4. Electrochemical measurements To prepare the electrodes for electrochemical testing in a threeelectrode system, samples derived from Chinese date were used as active materials, together with acetylene black as conductive additives and solution of polytetrafluoroethylene (1%) as binders, they were fully mixed and ground at a ratio of 8:1:1 in an agate mortar. The mixture was then evenly coated on a weighed nickel foam (2 cm*1 cm) using a doctor's blade, ensure that the coated area is 1 cm × 1 cm. The coated electrodes were needed to dry at 120 °C under vacuum overnight. The dried electrode sheets were then weighed again after being compressed (no thicker than 0.3 mm in thickness) to make sure that the mass loading of the active materials on each electrode was at least 3 mg. A CHI-660E electrochemical workstation (Shanghai Chenhua Instrucments Co.) was used to process the tests at room temperature. A Pt sheet was used as the counter electrode, and a Ag/AgCl electrode was used as the reference electrode, electrolyte was 6 M KOH aqueous. Electrochemical impedance spectroscopy (EIS) measurements were processed in a frequency range from 100 kHz to 0.01 Hz and the EIS data was analyzed by Nyquist plots. The specific capacitance (Cs, F/g) of the samples were calculated from the GCD curves using the following equations:

Cs =

I× t m× V

Scheme 1. Preparation process of DCt-ps.

1 1 1 × Cp V 2 2 4 3.6 E × 3600 P= t E=

where, Et (W h kg−1) is the specific energy density, Pt (W kg−1) is the specific power density, Cp (F g−1) is the specific capacitance based on the total device system, ΔV (V) represents the discharge voltage range exclusive of the IR drop and Δt (s) is the discharge time. All these electrochemical measurements were performed at least three times to eliminate as much error as possible and ensure the accuracy of the results.

(1)

3. Results and discussion

where I (A) is the charge-discharge current; Δt (s) is the discharge time, m (mg) refers to the mass loading of the active matters on electrodes; and ΔV represents the voltage change after the IR drop. For the two-electrode system test, all the electrochemical measurements were performed using CR2032 stainless coin type cells. Unlike the three-electrode system, the sample mixture was evenly coated on pieces of round nickel foam (1.0 cm in diameter) and pressed (0.3 mm in thickness, mass of the active materials on each electrode was at least 4 mg) after vacuum dried. A CR2032 stainless coin type cell was then assembled symmetrically in a glove box filled with argon using two of the dried electrodes with the closest amount of active materials as both of the working electrodes and reference electrodes, glass paper was used as the separator, and 1 M Na2SO4 or 1 M Et4NBF4 were used as electrolytes. Electrochemical impedance spectroscopy (EIS) measurements were also processed in a frequency range of 100 kHz to 0.01 Hz. Nyquist plots were used to analyzed the EIS data. The specific capacitance (Cp, F/g) of the symmetric supercapacitors were calculated using the following equations:

Cp =

4I × t m× V

3.1. Material characterization Scheme 1 concisely demonstrates the preparation process from Chinese date to porous carbon for high performance supercapacitors. As a plant with strong adaptability to the environment, date tree is widely distributed in northern China. The annual yield of Chinese date is high, far beyond the consumption of drugs and food processing, so it is a cheap and accessible raw material for preparation of carbon materials. October is the harvest season of Chinese date, at this time the content of all kinds of active ingredients is the highest. Therefore, we choose this season to harvest the Chinese date samples from Yan'An in Shaanxi, one of the main Chinese date producing areas of China. The cleaned and dried Chinese date was first calcined at 700 °C under the atmosphere of N2 for 2 h. In this stage, the polysaccharides and other components underwent dehydrated, aromatized and other complex changes, and then biochar DBC was formed. The DBC without obvious pore structures, so it was then further activated by KOH under different temperatures from 500 to 700 °C, with dosages of KOH various from 3 to 5 (KOH: DBC), finally obtained porous carbons DCt-ps, during this stage temperatures and activator dosages strongly affected the carbon yield, pore structures and the surface properties of the samples (Table 1). The morphology of the Chinese date derived samples were first analyzed by FESEM method, and the results are in Fig. 1. Fig. 1(a) to (f) is the SEM images of DBC and DCp-ts. After calcined under N2 at 700 °C for 2 h, the raw material has experienced a series of complex chemical and physical changes such as dehydration and aromatization, and finally formed the DBC as shown in Fig. 1a. The DBC has almost no

(2)

where I is the discharge current; Δt is the discharge time; m (mg) is the total mass loading of the active materials on both of the electrodes; and ΔV is the voltage change after the IR drop. The energy density and power density of a symmetric supercapacitors were determined by the following equations:

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Table 1 Carbon yield and pore structural characteristics of the DCt-ps. Samples

Carbon yield (%)

SBET (m2 g−1)

Smic (cm2 g−1)

Smes (cm2 g−1)

Vtotal (cm3 g−1)

Vmic (cm3 g−1)

Vmes (cm3 g−1)

Da (nm)

DC500-3 DC600-3 DC600-4 DC600-5 DC700-3

61 57 52 48 42

733 1299 1941 1421 1940

617 1141 1499 1156 1735

116 158 442 265 205

0.32 0.54 0.85 0.62 0.80

0.24 0.45 0.59 0.45 0.67

0.08 0.09 0.26 0.17 0.13

3.52 3.07 3.02 3.33 3.12

obvious pore structure, and the nearly solid surface is scattered with smaller carbon particles with irregular shapes. It can be speculated that during the carbonization process, high temperature causes melting, dehydration and carbonization of polysaccharides and other components, so that the internal skeleton structure of the raw material is damaged and reorganized, and some scattered particles are produced during the reorganized process. But without the etching action of an activator, the molten carbon resolidified into a whole after annealing, thus no obvious pore structure was formed and irregular carbon particles are scattered on the solid surface. With the participation of

activator KOH, the surface of the products obtained at different activation temperatures from 500 °C to 700 °C become fluffy (Fig. 1b, c, f). In addition, compared with the flat surface of DBC, the activated products show obvious irregular folds, these folds contribute to the formation of a large specific surface area, and there are almost no scattered carbon particles on the surface any more. As the temperature increased from 500 °C to 600 °C, some macroporous and mesopores were obviously formed on the surface of the activated product, and the surface became more irregular (Fig. 1c). However when the temperature further increased to 700 °C, the folds became more obvious (Fig. 1f), but

Fig. 1. SEM images of (a) DBC; (b) DC500-3; (c) DC600-3; (d) DC600-4; (e) DC600-5; and (f) DC700-3.

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Fig. 2. SEM images (a–d) and HRTEM images (e–i) of DPC600-4 at different magnifications.

the number of macropores and mesopores decreased compared with those of product activated at 600 °C. Surprisingly, when the ratio of the activator KOH was adjusted to 4 (KOH to DBC), the activated product presented an overall 3D wavy fold state as a whole, with a large number of cross-linked mesopores together with certain amount of macropores, and the surface of the product was covered with pits in regular shapes and sizes, and a large number of micropores were distributed all over the surface (Fig. 1d) [48–51]. 3 D Wavy folds, pits covered surface, and a large number of micropores provide the required large specific surface areas for electrolyte ion accommodation, while cross-linked mesopores enable ions to rapidly integrate into the surface of internal pores, thus facilitating high specific capacitance and multiplier performances. However, further increasing the ratio of activator to 5, the structure of the activated product changed dramatically (Fig. 1e). The three-dimensional structure is destroyed, and the whole structure is in an irregular and disordered state without obvious mesopores. According to the preliminary results of SEM analysis, the sample DC600-4 should have the optimal morphological architecture among the activated products. Therefore, SEM and HRTEM analysis at different magnifications were processed for further microscopic analysis of this sample (Fig. 2). The 3D architecture of the sample DC600-4 can be directly observed from Fig. 2a, the whole frame work is an hierarchical structure composed of macro-, meso- and micropores. The mesopores are cross-linked in the macropores, formed well-connected channels for

electrolyte ions (Fig. 2b and c). The surface of the fold is covered with cross-linked pits, which greatly increase the specific surface areas of the material (Fig. 2d). The internal structure of DC600-4 is further revealed by HRTEM. The intersecting and overlapping carbon layer inside the material is relatively thin (Fig. 2e and f), which means that the material has good electrical conductivity, and there are interconnected mesopores between layers, with a few macropores distributed (Fig. 2g and h) however mainly composed of a large number of micropores (Fig. 2i). The pore volumes and pore size distributions were tested using nitrogen adsorption-desorption method. Fig. 3(a) is the N2 adsorptiondesorption isotherms of the samples. According to the IUPAC classification, all the samples display typical type I isotherms. At a relative pressure (P/P0) of below 0.1, the amount of nitrogen adsorbed by all the samples increase sharply, this proves that DCs is mainly composed of abundant micropores [52]. The nitrogen adsorption capacity of the sample DC600-4 is the largest, which indicates that the specific surface area of this material is the largest. The adsorption amount of nitrogen increased slightly after the relative pressure was greater than 0.1, indicating the existence of a small number of mecropores. In comparison, the adsorption amount of DC600-4 increased most obviously, which proved that the material contained more mesoporous. Careful observation shows that DC600-4 has a very short tail at a relative pressure of 0.97–1.0, which proves that the material also contains trace amount of macropores [53]. The pore size distributions based on the DFT

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Fig. 3. (a) Nitrogen adsorption-desorption isotherms (b) pore size distributions (c) Raman spectra (d) XRD patterns of DCt-ps.

method are showed in Fig. 3b, the pore sizes of all the samples were concentrated at 0.5–2 nm, and a small amount of mesoporous less than 4 nm were exist, and the mesopores content of DC600-4 was relatively the highest. The results of carbon yields, specific surface areas and pore structures characteristics of samples are summarized in Table 1. With the increase of the activation temperature, the specific surface areas of the activated product also increase. When the activation temperature increased from 500 °C to 700 °C, the specific surface area increased from 733 m2 g−1 to 1940 m2 g−1. However, with the increase of the dosage of activator, the specific surface area of the activated products increased first and then decreased. The change of pore volume is the same as that of specific surface area. Therefore, the regulation of activation temperature and dosage of activator has a great influence on the structures of DCt-ps. Finally, among the activated samples, the sample DC600-4 has the maximum specific surface area of 1941 m2 g−1 with the highest pore volume of 0.85 cm3 g−1. It can be seen that DC6004 has the maximum specific surface area and pore volume, as well as a three-dimensional structure with coexistence of macropores, mesoporous and micropores. It basically has the characteristics required by an ideal supercapacitor material, so the sample DC600-4 is expected to have the best performance for supercapacitors. It is worth mentioning that both increase the activation temperatures and dosages of activator, the carbon yields are decreased. Raman spectrums of the samples are in Fig. 3c. There are two obvious peaks located at 1350 cm−1 and 1590 cm−1 for all the samples correspond to G and D band respectively [54]. The G bands related to the degree of graphitization while the D band is associated with local defects and disordered properties of the DCt-ps samples [55]. The ratio value of IG/ID is related to the degree of structure ordering and graphitization. The IG/ID values of the samples are 1.04, 1.02, 1.02, 1.00, 1.00, corresponding to DC500-3, DC600-3, DC600-4, DC600-5 and DC700-3, XRD patterns are showed in Fig. 3d, all the samples showed a obvious wide peak at ~23.32° and ~43.8°, correspond to the (100) and 101 crystalline plane of graphite, these mean their certain degree of

graphitization, which is benefit for electricity conductivity [56] of the DCt-ps. Heteroatom doping is one of the common strategies to prepare highperformance supercapacitor carbon materials [57–59]. Chinese date is rich in a variety of active ingredients, so it is inferred that the derived carbon materials should have the presence of self-doped heteroatoms. To verify this inference and study the effect of activation conditions on heteroatoms content, the elements in DBC and DCt-ps were analyzed by XPS (Fig. 4). Fig. 4a is the full spectrum of XPS. It can be seen from the figure that there are three peaks on the spectrum at 287 eV, 375 eV and 532 eV, corresponding to C1s, N1s and O1s respectively [60]. It is proved that DBC and DCt-pS are composed of C, N and O elements [58]. Peak intensities of C1s and O1s in all samples are strong, however, the peak intensity of N1s only exists weakly in DBC, suggests a high content of C and O and a lower content of N in all the samples. The contents of these elements in the samples are summarized in Table S1. According to the results of element content analysis, the content of nitrogen in DBC was the highest (1.08 at.%), and this content decreased sharply with the participation of activator. With the increase of activation temperature and dosage of activator, the content of nitrogen decreased correspondingly, indicating that the stability of nitrogen to temperature and KOH activator was poor. Interestingly, there was no linear correlation between the content of O and C and the activation conditions. In addition, the content of C is the highest and the content of O is the lowest in the sample DC600-4 among the DCt-ps. The high-resolution C1s spectrum is resolved in to three independent peaks, located at 284.7, 285.5 and 286.6 eV, corresponding to C]C, CeO, and C]O respectively [61]. Three individual peaks located at 531.5, 532.3 and 533.2 eV can be deduced from the high-resolution O1s spectra, these three peaks corresponding to the OeI (adsorbed O), O-II (C=O/C-O-C), and O-III (O-C=O) type oxygen respectively [62,63], and also no obvious NeO was detected.

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Fig. 4. (a) Full XPS survey spectra, (b) high-resolution C1s spectra, (c) high-resolution N1s spectra, and (d) high-resolution O1s spectra of DBC and DCt-ps.

3.2. Electrochemical performance

(Fig. S1). The specific capacitances of the samples at different current densities are summarized in Fig. 5e, which obviously demonstrate that specific capacitance and rate stability of the sample DC600-4 are the best among them. Nyquist plots of the samples are tested in a frequency ranged from 0.01 Hz to 10 kHz. Just in good agreement with the CV and GCD tests, the sample DC600-4 exhibited lower electrolyte resistance and charge resistance according to the semi-circle segment. The short slop of the Warburg segment means quick diffusion or translate of ions [17], and finally the near vertical part at low frequency verifird the EDLC characteristics. These results are just as we excepted before, for the unique 3D structure of the sample DC600-4, as well as the coexistence of macropores, mesopores and micropores, together with self-doped of O, finally enabled it to exhibit high performance as electrode for supercapacitors [57]. The prominent performances of DC600-4 have been preliminarily verified in the three-electrode system in 6 M KOH. Next, to verify the practical application value of this material in the devices, symmetrical coin type supercapacitors were assembled, and the electrochemical tests were also evaluated (Fig. 6). The tests were all processed for three times. In two-electrode systems using KOH as the electrolyte, the high voltage can be enlarged to 1 V. It can be observed from the shapes of CV and GCD curves that the EDLCs characteristics of DC600-4 based electrodes are well maintained in assembled devices (Fig. 6a, b). High specific capacitances of 275 F/g was generated at a current density of 0.5 A/g, and 125 A/g was retained at a current density of 10 A/g calculated based on the GCD curve (Fig. 6b). After cycled at a current density of 10 A/g for 10,000 cycles, the specific capacitance was then retested at a current density of 0.5 A/g, and 86.7% of the original capacitance was retained. These results further verified the excellent performance of DC600-4 as electrode for real supercapacitors. However, the high voltage is limited to 1 V, restrict its applications, what's more, according to the Ragone plot, the energy density delivered in this KOH based system is not satisfactory, the highest energy density was only 9.1 Wh kg−1 at a power density of 207 W kg−1, though this is higher

The electrochemical properties of the samples were first tested in a three-electrode system using 6 M KOH as the electrolyte; the results all tested three times showed in Fig. 5. Fig. 5a shows the CV curves of the DCt-ps tested at a scan rate of 200 m Vs−1. The CV curves all possess relatively regular and symmetrical spindle shapes, without obvious redox peaks, indicating typical ideal EDLC characteristics [52], and no apparent pseudocapacitance is detected, which means the nitrogen content in the material is too low to trigger pseudocapacitance. The CV areas of DC500-3 and DC700-3 are much smaller than those of the other three samples, indicating smaller specific capacitances. Of which, DC600-4 based electrode exhibited the largest CV area, inferring the highest specific capacitance, just as we expected. The CV curves of DC600-4 at different scan rates vary from 5 to 200 mV/s are measured to verify the performance of this sample, and the results are in Fig. 3b. With the scan rate increased from 5 mV/s to 200 mV/s, the CV curves well maintained their symmetric spindle-like shapes, demonstrated the excellent transport ability of the material to electrolyte ions, due to the existence of mesopores and macropores together with co-doped of O and N, just as we anticipated before. The GCD curves of the samples are showed in Fig. 5b. The results of GCD tests at a current density of 0.5 A/g are in good agreement with those of the CV curves, specific capacitances of electrodes based on DC600-3, DC600-4 and DC600-5 are 497.5 A/g, 518 F/g and 367 F/g respectively, much higher than those of DC500-3 and DC700-3, which is 248.5 F/g and 287 F/g. Benefit from its excellent unique structure, the specific capacitance of sample DC600-4 is almost among the highest of those of porous carbons derived from biomass ever reported. And the electrode base on DC600-4 also exhibited great rate stability as showed in Fig. 5d, at a current density of 0.5 A/g, the specific capacitance is 518 F/g, at a high current density of 10 A/g, the specific capacitance is as high as 400 F/g. What is particularly surprising is that even at a high current density of 70 A/g, the specific capacitance can still be 40 F/g 7

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Fig. 5. Electrochemical performance of DCt-ps in a three-electrode system in 6 M KOH (a) CV curves of DCt-ps at a scan rate of 200 mV/s; (b) CV curves of DC600-4 at different scan rates vary from 5 to 200 mV/s; (c) GCD curves measured at a current density of 0.5 A g−1; (d) GCD curves of DC600-4 at various current densities from 0.5 A/g to 70 A/g; (e) specific capacitance of DCt-ps at different current densities; (f) Nyquist plots of the samples.

than those of the commercial carbon based electrode materials (normally 4–6 Wh kg−1) [17], however, much lower than those of the biomass derived porous carbons we mentioned in the introduction part [27,32,35,36]. In addition, according to the unique structure and performances of DC600-4 in the three-electrode system, the supercapacitor based on this material should be able to deliver wider voltage window and higher energy density. Therefore, further strategies are needed. Just as we mentioned before, according to the equation of Energy density E= 1 CΔV2, energy density is depended on capacitance and the 2 voltage window together [35]. Of which the capacitance mainly depend on the electrode materials, while the voltage window is determined by the electrolytes, for the former, we have already obtained high capacitance porous carbon DC600-4, thus, the next step is to select proper electrolytes. Sodium sulfate and organic salts are commonly used to wilder the voltage window of a supercapacitor system. The former is cheap and easy to operate, but only up to 2 V voltage is available, the latter is complex and expensive to prepare, but up to 4 V voltage can be provided [4], each has its own advantages and disadvantages. However our purpose is just to verify the properties of our material DC600-4, therefore we have chosen both electrolytes for further study respectively. DC600-4 based symmetric supercapacitors using 1 M Na2SO4 and 1 M

Et4NBF4/AN electrolyte were assembled and tested respectively. Fig. 7a and b are CV curves at a scan rate of 20 mV/s with different voltage windows of as-fabricated symmetrical supercapacitors in 1 M Na2SO4 and 1 M Et4NBF4/AN. In 1 M Na2SO4, when the voltage is below 2 V, from 0 to 1.8 V, CV curves well maintained a good rectangular shape, it means that the system has excellent double layer performance and reversibility. When the voltage is further enhanced to 2 V, current density increased sharply at the anode scan, this means discomposition of the electrolyte at this voltage [17], thus the high voltage in Na2SO4 is 1.8 V (Fig. 7a). For the Et4NFB4/AN, the phenomenon is similar, except that electrolyte discomposition occurs at a voltage of higher than 3 V, thus 3 V is selected as the work voltage (Fig. 7b). GCD tests under voltage of 1.8 and 3.0 V were then processed at various current densities to evaluate the capacitance of DC600-4 based supercapacitors in 1 M Na2SO4 and 1 M Et4NFB4/AN electrolytes respectively. The GCD curves under different current densities all present isosceles triangle shapes, indicating good EDLCs characteristics [123]. And the IR drops are very low, which means lower resistances and good ion diffusion and transport abilities. Specific capacitance tested at a current density of 0.5 A/g are 164 F/g and 168 F/g, and at a current density of 20 A/g they are decreased to 66 F/g and 99 F/g, with capacitance retention of 40.2% and 58.9% respectively. The specific 8

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Fig. 6. Electrode performances of DC600-4 based symmetric supercapacitor using 6 M KOH as electrolyte (a) CV curves at different scants from 0.5 to 200 mV/s. (b) Galvanostatic charge-discharge curves processed at a current density of 0.5 A g−1 to 10 A/g. (c) Cycling stability measured at a current density of 0.5 A g−1 after cycled at 10 A/g for 10,000 cycles. (d) The specific capacitances of symmetrical supercapacitor at different current densities and the insert is the Ragone plot.

capacitance as a function of current density is calculated based on GCD curves is shown in Fig. 7e, It can be seen that the specific capacitance and multiplier properties of DC600-4 based electrode in organic electrolyte are both higher than those in aqueous electrolyte, though they are much lower than those in KOH electrolyte, may the attribute to the smaller size of OH−. However, benefit from higher voltages of Na2SO4 and Et4NBF4/AN, the energy density is greatly enhanced to 18.5 Wh kg−1 at a power density of 373.8 W kg−1 in the aqueous electrolyte Na2SO4, and a remarkable high specific energy density of 51.3 Wh kg−1 was achieved at a high power density of 767.8 W kg−1 in Et4NBF4, nearly twice and five times than that of in 6 M KOH according to the Ragone plots in Fig. 7d. And the high energy density of 51.3 Whkg−1 at a high power density of 767.8 W kg−1 in Et4NBF4 is among the highest energy density of biomass derived porous carbons used as electrode for supercapacitors (Table S2). Nyquist plots of impedance from 0.01 Hz to 100 kHz were also measured to evaluate the electronic conductibility of DC600-4 in Na2SO4 and Et4NBF4 electrolyte (Fig. 8a). The near vertical lines in the low frequency for both of them verified their EDLCs properties and high performance capacitance, also indicating them with good durable abilities. The low equivalent series resistances are calculated to be 0.75 Ω and 1.25 Ω respectively, of which resistance in Et4NBF4 is a little bit higher than that of in Na2SO4. And the short of the 45° segments indicate DC600-4 with lower resistance of ion translation and high efficient of ion diffusion for both the aqueous and organic electrolyte. Thus verified the good electronic conductivity of DC600-4. Cycle stability was conducted at a current density of 20 A/g for 10,000 cycles respectively. After cycled for 10,000 cycles, the specific capacitance of symmetric supercapacitor in 1 M Na2SO4 without obvious decrease with a capacitance retention of 98.9%, while the capacitance retention decreased to 90.2% for in 1 M Et4NBF4, and the obvious decrease started when cycled for 6000 cycles, This may be

attributed to the poor stability of the organic electrolyte. During the charging and discharging process, partial electrolyte discomposed occurs, which leads to increased resistance and thus reduces the capacity. But overall, after 10,000 cycles, the capacity retention rate of 90.8% is still excellent. The assembled coin type symmetric supercapacitor using DC600-4 as the electrode material and 1 M Et4NBF4 as electrolyte was then used to motivate a LED bead with activation voltage of 3 V. After charged under 3 V for 5 min, the supercapacitor successfully lighted the LED bead for 0.5 h, verified the high performance of DC600-4 as electrode material for supercapacitors. 4. Conclusions Through a simple two-steps process, we have successfully turned an abundant biomass Chinese date in to 3D porous electrode material for high performance supercapacitors with high capacitance and energy density. The porous carbon prepared under the optimized conditions mainly composed of micropores and a certain number of mesopores and macropores, together with self-doped O (18.3 at.%) and N (1.83 at.%) elements. The mesopores and macropores are conducive to the rapid transfer and diffusion of the electrolyte ions, while the existence of micropores and irregular pits contribute to the large specific surface areas to provide more reactive sites for ions, and the participation of heteroatoms especially O promote the wettability of electrolyte to electrode materials. It is just because of a combination of these advantage factors together, thus the electrodes based on this material generated an ultrahigh specific capacitance of 518 F/g at 0.5 A/g, with excellent rate performance of 40 F/g at 70 A/g. Notably, symmetric supercapacitors based on this material exhibited high energy density of 18.5 Wh kg−1 at a power density of 373.8 W kg−1 in Na2SO4, and a remarkable high specific energy density of 51.3 Wh kg−1 at a high power density of 767.8 W kg−1 in the organic electrolyte Et4NBF4. 9

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Fig. 7. Electrode performances of DC600-4 based symmetric supercapacitors using 1 M Na2SO4 and 1 M Et4NBF4/AN electrolyte. (a) CV curves at a scan rate of 20 mV/ s with different voltage range at 1 M Na2SO4. (b) CV curves at a scan rate of 20 mV/s with different voltage range at 1 M Et4NBF4/AN. (c) GCD curves at different current densities at 1 M Na2SO4 at 1.8 V; (d) GCD curves at different current densities at 1 M Et4NBF4/AN at 3 V (e) The specific capacitances of symmetrical supercapacitors at different current densities using Na2SO4 and Et4NBF4/AN electrolyte; (f) Ragone plots of the DC600-4 based supercapacitors in different electrolytes.

Fig. 8. (a) Nyquist plots of impedance from 0.01 Hz to 100 kHz. (b) Cycling capabilities tested at a current density of 20 A/g for 10,000 cycles; one DC600-4 based coin type supercapacitor using Et4NBF4 electrolyte motivate a LED bead with activate voltage of 3 V (insert).

These excellent results verified the potential of Chinese date derived porous carbon for high performance supercapacitors, and due to the advantages of abundant, renewable and inexpensive raw material,

together with eco-friendliness simplicity preparation process, it is practical to industrialization this material in the near future for scale-up production of high energy density supercapacitors. 10

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Acknowledgments

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